ISL6721EVAL3Z Intersil, ISL6721EVAL3Z Datasheet - Page 5

• Manufacturer Part Number: ISL6721EVAL3Z
• Description: EVAL BOARD 3 FOR ISL6721
• Manufacturer: Intersil

Specifications of ISL6721EVAL3Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Feedback, OCP and Slope Compensation
Slope compensation is a technique to add additional
slope to the current signal for current mode PWM with
duty ratio larger than 50% to improve noise immunity.
The downslope required is assumed to be 150mV during
the off time, for a slope compensation corresponding to
the maximum duty cycle of 42% (see Equation 18).
The amount of voltage that needs to be added to the
current sense signal is given by Equation 19.
From the datasheet, the minimum capacitance required
for slope compensation is shown in Equation 20.
An appropriate value for slope compensation capacitance
would be between 1/2 and 1/3 of the calculated
minimum value, giving a range between 36pF and 53pF.
The effective slope voltage added to the current sense
signal can be calculated according to Equation 19, which
is about 160mV with the equivalent capacitance 47pF.
The current level that corresponds to the overcurrent
threshold must be chosen to allow for the dynamic
behavior of a wide-input voltage range converter. The
current limit threshold, ISET was set using a simple
resistor divider at 1.07V, which is near its maximum
value, in order to minimize noise effects. Setting the
current limit to about 130% of maximum output power,
corresponding to maximum duty cycle or low line, the
current sense components can be calculated as follows in
Equation 21.
Selecting a current sense transformer with a turns ratio
of 1:50, the burden resistor needs to be set, taking into
V
Transformer
Rectifiers
Primary MOSFET
Chokes
Snubber
Fixed Control Estimate
Capacitors and CST
Total Loss Estimate
Downslope
C
slope
slope
=
min
Downslope
-------------------------------- -
TABLE 1. POWER LOSS ANALYSIS
=
=
DEVICE
4.24
----------------------- -
2
V
150mV F
----------------------------------- -
slope
(
1 D
×10
–
6
•
max
•
•
D
-------------- -
F
D
-------------- -
max
sw
F
sw
)
max
sw
=
5
=
=
150mV 300
----------------------------------------------- -
1
-- -
2
4.24
----------------------- -
•
55mV
------------------
1
(
1 0.42
×10
POWER DISSIPATED
78
×10
–
•
–
3
6
•
•
)
(WATTS)
--------------------- -
300
--------------------- -
300
×10
0.42
0.42
Application Note 1491
7.14
2.26
1.68
1.13
0.81
0.69
0.07
0.5
3
×10
×10
≅
78
3
3
≅
≅
mV
-------- -
(EQ. 18)
μs
55mV
108pF
(EQ. 19)
(EQ. 20)
account the additional voltage on the current sense
signal induced by the slope compensation s shown in
Equation 22.
From Equation 17, V
The current sense resistor can be selected according to
Equation 23.
A 6.49Ω, 1% resistor can be used. The current sense
signal is filtered by means of an RC filter, with a detection
time constant of about 100ns.
The small signal model for multiple-output current mode
control and the compensation network design is available
in the ISL6721 datasheet. The feedback of the two
outputs has been weighted at 80-20% between the 12V
and 18V, to provide the 12V rail with the tighter
regulation.
I
OCP
V
R
ISET
sense
=
=
I
in
=
(
peak
V
V
---------------------------------------------
ISENSE
ISENSE N
•
I
1.3
OCP
•
•
=
0.8
I
------------------------------------------- -
ISENSE
CST
inRMS
)
+
0.1
D
=
max
max
1.015 50
-------------------------- -
+
needs to be about 1.015V.
V
7.8
•
slope
1.3
•
=
=
2.58 1.3
-------------------------
6.5Ω
0.43
•
=
7.8A
(EQ. 22)
(EQ. 21)
(EQ. 23)
AN1491.0